I. Field of the Invention
This invention relates to temperature measuring devices, especially thermocouples, used for directly measuring the temperatures of hot moving highly abrasive particulate materials. More particularly, the invention relates to such devices which can be used effectively in harsh conditions in industrial equipment such as rotary calcining kilns or other furnaces.
II. Discussion of the Prior Art
Rotary calcining kilns are used in industry to heat and calcine various materials, such as petroleum coke and alumina, and it is usually beneficial to control the temperature profile within such kilns quite carefully to ensure that a valuable product is consistently produced. The most direct way of controlling the temperature profile would be to measure the temperature of the material being calcined at various positions spaced along the interior of the kiln and then to modify one or more of the factors which affect the temperature profile, such as the rate of rotation of the kiln, the rate of raw material feed, the heat input, the oxygen input, the angle of inclination of the kiln, and the like. However, this has proven to be difficult or impossible in practice because the conditions prevailing within such kilns are extremely hostile to direct temperature measuring devices, such as thermocouples, and such devices are so rapidly damaged or destroyed and that their use has been largely impractical.
Instead, resort has been made to indirect (non-contact) temperature measurement apparatus, e.g. optical pyrometers and even television cameras to observe such temperature-related characteristics as the positions of fireballs within the kiln or the physical appearance of the material being treated at various points. However, this has not proven to be entirely satisfactory. For instance, Canadian patent no. 1,052,313 issued to Merlyn M. Williams on Apr. 10, 1979 and assigned to Alcan Research And Development Limited, describes the difficulty of using optical pyrometers to measure temperatures within rotary calcining kilns, particularly in the presence of smoky discharges of kiln gases that can lead to false readings. The solution to this problem, according to this patent, is to provide an additional optical pyrometer spaced from the discharge end of the furnace and to operate an air supply when the output from the additional pyrometer becomes unstable in order to prevent smoky discharge from affecting the output of the additional optical pyrometer. This solution, while effective for overcoming the problem to be solved, merely illustrates the need for a more direct and reliable way of measuring temperatures within such kilns and furnaces by means of temperature sensors that can directly contact and measure the temperature of the charge within the kiln or furnace.
Attempts have been made to provide temperature measuring devices with protective coatings or casing that can resist the harsh conditions. However, such coatings and casings, while being able to resist very high temperatures and contact with moving abrasive particles, must not unduly reduce the sensitivity and accuracy of the temperature measuring devices and it has been difficult to find a material that combines suitable properties such as toughness, heat shock resistance, physical shock resistance and wear resistance with appropriate thermal conductivity and low thermal mass.